Steam generator tubes of nuclear power plants are a heat exchanger which transfers heat from the primary coolant loop to the secondary one to produce steam in the latter. At an early stage of the nuclear industry, Alloy 600 was mostly used as steam generator tubes but with increasing plant operation time, Alloy 600 is well-known to be very susceptible to primary water stress corrosion cracking (PWSCC). To overcome this problem, Alloy 690 containing a higher content of Cr than Alloy 600 has recently been used as steam generator tubes, instead of Alloy 600, because Alloy 690 is well-known to be much higher resistance to PWSCC.
Alloy 600 is a Ni-base alloy with a composition in weight percent of 14-17% Cr, 6-10% Fe, 0.15% C max., 1% Mn max., 0.5% Si max., and 0.015% S max., and Alloy 690 is a Ni-base alloy with a composition in weight percent of 27-31% Cr, 7-11% Fe, 0.05% C max., 0.5% Mn max., 0.5% Si max., 0.5% Cu max., and 0.015% S max.
As described above, Alloy 690 is a material with a higher Cr concentration than Alloy 600, which was called “Inconel Alloy 690,” after the name of the developer, or the Inco Alloys International. Inc. but is now called “Alloy 690” due to the expiration of the patent. Since Alloy 690 has a lower thermal conductivity by 11% than Alloy 600, a replaced steam generator made of Alloy 690 should contain a higher number of Alloy 690 tubes by 11% to compensate the loss of thermal heat transfer caused by a lower thermal conductivity of Alloy 690, leading to an increase in the size of a steam generator tube of Alloy 690 and in the manufacturing cost.